1. Field of the Invention
This invention relates to a mechanism for applying a preload to a bevel gear device, and more particularly to improvements in a mechanism for applying a pre-load to bevel gear device suitable for use in a steering system of a cab-over type motor vehicle, wherein bevel gears are provided which are fixed to forward ends of shafts crossing each other, respectively, and in mesh with each other, and a turning force applied to one of the shafts is imparted to the other.
2. Description of the Prior Art
As shown in FIG. 3 for example, in general, the steering system of the cab-over type motor vehicle is comprised of: a steering shaft 12 for supporting a steering wheel 10; a rack and pinion type steering device 14 for steering front wheels, not shown, for example; and a bevel gear device 22 for connecting the steering shaft 12 to the steering device 14 through couplings 16, 18 and 20.
In the bevel gear device 22, bevel gears 28 and 30, which are fixed to shafts 24 and 26 crossing each other, respectively, and in mesh with each other, and a turning force applied to one 24 (hereinafter referred to as an "input shaft") of the shafts is imparted to the other 26 (hereinafter referred to as an "output shaft").
In the bevel gear device 22 used in the steering system of the motor vehicle as described above, in order to set the efficacy and return of the steering wheel suitably, it is necessary to make the pre-load applied to the bevel gears 28 and 30 compatible with the backlash. More specifically, when the backlash is excessively tightened, the pre-load is increased, whereby, as shown in FIG. 4 for example, fluctuations in mesh between the gears by fluctuations C of the backlash cannot be absorbed, so that an excessively large tooth surface force occurs. On the other hand, when the backlash is excessively high, a play in the steering system becomes excessively large, thus presenting such problems that the steering wheel may become unsteady, impact sounds of the tooth surfaces due to an external input such as a road surface, etc. occur.
As a consequence, it is necessary to give a predetermined suitable backlash. However, there is the limit due to the manufacturing accuracy and the assembling accuracy of the gears. Furthermore, it is difficult to strictly adjust the backlash.
On the other hand, as a pre-load adjusting mechanism for applying a suitable pre-load to bevel gears in the steering system of a motor vehicle, as described in Japanese Utility Model Laid-Open (Kokai) No. 160873/1983, there is provided that a pre-load block is brought into pressing contact through a resilient force with a small gear end face of the bevel gear 28 on the side of the input shaft 24 connected to the steering wheel 10, so that the steering wheel 10 can be prevented from being unsteady.
Furthermore, a mechanism for removing a backlash from bevel gears, as described in Japanese Utility Model Laid-Open (Kokai) No. 125760/1983, there is proposed that at least one of the bevel gears is journalled on the shaft of the bevel gear in a manner to be movable at least in the axial direction, the bevel gear and the shaft are connected by a sheet spring disposed substantially perpendicular to the axial direction of the shaft, whereby this bevel gear is urged against the other bevel gear, so that the backlash can be prevented without impairing the positioning accuracy.
However, the sole object of the pre-load adjusting mechanism proposed in Japanese Utility Model Laid-Open (Kokai) No. 160873/1983 is to make adjustable the pre-load applied to the input shaft 24 on the side of the steering wheel 10, and presents such a problem that the fluctuations in backlash between the bevel gears 28 and 30 cannot be absorbed because both the input shaft 24 and the output shaft 26 are fixed in the axial direction.
On the other hand, in the backlash removing mechanism proposed in Japanese Utility Model Laid-Open (Kokai) No. 125760/1983, the fluctuations of the backlash can be satisfactorily absorbed and the positioning accuracy is high, however, such an arrangement is adopted that the shafts and the bevel gears are connected to each other through a Belleville spring, so that a high turning torque cannot be transmitted, disadvantageously.
Furthermore, in each of the Japanese Utility Model Laid-Open (Kokai) Nos. 160873/1983 and 125760/1983, since the direction of biasing the bevel gears for applying the pre-load or removing the backlash has been single, such a disadvantage has been presented that it is difficult to make the tooth surface force applied to the bevel gears compatible with the positioning accuracy for bearings, etc. for rotatably supporting the bevel gears.